The use of a downhole motor to drive a rotating tool, for example a milling tool, in a downhole assembly has well recognized advantages. Downhole motors available heretofore have, however, suffered from the disadvantage that the rotor of the motor cannot be locked to the stator of the motor. As a result, relative rotation of the elements of a tool assembly above and below the motor is possible. This renders impossible or at least complicates the accurate angular orientation of the components located below the motor.
If the components below the motor include a hydraulically settable packer or anchor, the fluid displacement required to set the packer or anchor is liable to drive the motor during the setting procedure, further complicating accurate angular orientation of the tool.
In certain applications, for example the drilling of a lateral well bore from a main well bore using a whipstock to deflect a milling tool, accurate angular orientation of certain components (the whipstock in this case) is critical.
A prior art lockable motor assembly is disclosed in FR-A-2,332,412. This prior art motor assembly comprises a locking member which is initially fixed, by means of a shear pin, relative to a motor stator at a location axially spaced from the motor rotor. The arrangement is such that the rotor may freely rotate relative to the stator. In the event that the rotor is to be locked relative to the stator, a drop ball is released into the motor assembly and received on a shoulder within a bore of the locking member. The position of the drop ball within the bore of the locking member allows a fluid flow within the motor assembly to apply sufficient force on the locking member to shear the shear pin and move the locking member into engagement with the rotor. The rotor is thereby rotationally fixed relative to the stator. The prior art motor assembly does not comprise means for returning the locking member to a position where the rotor is free to rotate relative to the stator.
A further prior art motor assembly is disclosed in U.S. Pat. No. 4,705,117. This prior art motor also comprises a rotor lockable relative to a stator by means of a locking member. During use of the prior art motor, the locking member is initially located so as to prevent rotation of the rotor relative to the stator. The locking member is rotationally fixed relative to the stator and secured to the rotor by means of a plurality of shear pins. In the event that the rotor is to be rotated relative to the stator, a drop ball is released into the motor assembly and received by the locking member. As a consequence, passageways defined in the locking member are closed by the drop ball allowing a fluid flow within the motor assembly to apply sufficient force on the locking member to shear the plurality of shear pins and move the locking member into a position whereby both the rotor and the locking member may rotate relative to the stator.